Electric arc furnaces are used inter alia for melting scrap metal. Typically, several tons of scrap are dropped onto the hearth of the furnace. The furnace is then closed with a cover which is provided with several downwardly extending electrodes. When a voltage is applied between the electrodes the scrap melts forming a pool of molten metal in the hearth.
Typically, the scrap will be contaminated with paint, grease, oil and other carbonaceous material and these can conveniently be oxidized by inserting a lance into the furnace and blowing oxygen through the lance.
Because the heating is very localized it is also very advantageous to stir the molten metal to ensure that the temperature of the molten metal is substantially uniform throughout.
Rather than using a lance and electromagnetic stirring it has been proposed to introduce oxygen through tuyeres in the bottom of an electric arc furnace for both the oxidation procedure and stirring and then replace the oxygen with inert gas to continue the necessary stirring when the oxidation procedure is complete.
Whilst this technique has worked well with oxygen-steelmaking furnaces which tend to have a deep pool of molten metal and tend to be tall and tulip shaped it has proved difficult to adapt this technique to electric arc furnaces. In particular, the pool of molten metal tends to be relatively shallow with the result that, using conventional techniques, the explosive expansion of the oxygen and argon creates a fountain of molten metal up to 15 m in height. Droplets of this molten metal solidify between the side wall and the cover making it necessary to melt them with a lance before the cover can be removed.
If the flow of oxygen or inert gas is reduced then molten metal can enter the tuyeres, solidify and render them inoperative.
In order to avoid this it has been proposed to provide the bottom of the hearth with tuyeres of relatively narrow diameter, e.g., of the order of 1.7 mm internal diameter and to blow oxygen and inert gas through the tuyeres so that it leaves the tuyeres at at least sonic velocity. Although the gas emerges from the tuyeres at such high speed the volume is such that the formation of fountains is minimal.
One of the major problems associated with this solution is that argon and oxygen are generally only available at about 16 bar A (absolute) or somewhat less from normal commercial industrial gas tanks. The tuyeres pass through the bottom of the furnace to a main and are typically about 1.7 mm in diameter and 1000 mm long. Because of the restriction offered by the tuyere it is not possible to achieve the desired gas throughput at at least sonic velocity with normal gas supply installations. To counteract this problem one commercial installation includes a compressor and a battery of storage vessels constructed to contain oxygen and argon at 26 bar A. It will be appreciated that the cost of this additional equipment is very high. An alternative solution has been to use high pressure storage vessels and to pressurize them to the desired level. Such high pressure vessels are also very expensive.
The aim of at least preferred embodiments of the present invention is to provide a simple and inexpensive alternative to the prior art described.